1. Field of the Invention
This invention relates to tip caps for rotor blades, and particularly to a new and improved tip cap which is effective for cleaning the shroud surrounding the rotor assembly as well as for providing a close-clearance seal between the rotor blade and the shroud.
2. Description of the Prior Art
The rotor blades of a rotor assembly in a gas turbine engine are normally surrounded circumferentially by a shroud. The purpose of the shroud is to prevent gas, flowing through the portion of the engine containing the rotor assembly, from bypassing the rotor blades. Without the shroud, the gas could flow outwardly of the radially outer end, or tip, of the rotor blade. The energy of that gas which is prevented from bypassing the rotor blades is utilized to help rotate the rotor assembly. Therefore, engine efficiency increases as the amount of gas bypassing the rotor blades decreases.
To decrease the amount of gas escaping between the tip of a rotor blade and the shroud, the gap between the tip of the rotor blade and the shroud should be minimized as effectively as is practical. One method which is used to minimize the gap is to fabricate the rotor blade to be of such a radial length that the radially outer end, or tip, of the blade is disposed closely enough to the inner surface of the shroud so as to form a seal by itself. Problems can arise when this method is used, however, primarily due to the effects of rubbing. Rubbing is contact between the blade tip and the shroud. Rubbing can be caused by, among other reasons; thermal expansion and contraction of the rotor blades and the shroud, the shroud being not perfectly round, the rotor blades being of different lengths, or deposits of metal or other materials on the shroud or the blade tip.
Rubbing is disadvantageous in that it reduces engine efficiency by converting rotational energy of the rotor assembly into heat resulting from rubbing friction. Rubbing is also disadvantageous in that the tip of the rotor blade is worn away by rubbing. The tip material which is worn away is often deposited on the inner surface of the shroud and, as a result, can eventually cause the other blade tips to rub. Still another disadvantage of rubbing is that the blade tip which rubs is subject to structural fatigue, such as cracking, because of thermal stress due to friction and shear forces due to contact between the blade tip and shroud. Thus, when the tip of a rotor blade is subject to rubbing, the useful life of the blade tip, and thus the engine rotor blade, is shortened. Rubbing, therefore, causes the rotor blade to be replaced sooner than it would be in the absence of rubbing. Blade replacement as a result of wear due to rubbing constitutes a large cost to the user.
One means for reducing the disadvantageous effects of rubbing is the utilization of tip caps on rotor blades. A tip cap is a relatively small extension, having a cross-sectional shape conforming to that of the rotor blade, and which is either integral with or mounted on the radially outer end of the rotor blade. Such a tip cap is also sometimes referred to as a "squeeler tip cap" or a "squealer", but will be referred to simply as a "tip cap" hereinafter. A tip cap which rubs is subject to being worn away and is subject to the same thermal and shear stresses as is a blade tip which rubs. However, if the tip cap can be made to be replaceable, then only the tip cap itself, rather than the entire rotor blade, need be replaced, resulting in a great reduction in cost to the user.
Most tip caps are made of metal. As such, they leave metallic wear deposits on the inner surface of the shroud when they rub. As mentioned earlier, such deposits cause further rubbing to occur. Also, the tip caps become heated due to metal-to-metal friction between the tip cap and the shroud which is also metal. The resultant thermal stresses shorten useful tip cap life by causing fatigue and cracking in the tip cap. Many currently used tip caps include cooling arrangements therein to reduce thermal stresses. However, rotor blades with such tip caps still require relatively frequent replacement or refurbishment because of the inadequacy of the tip cap cooling arrangements and the other aforementioned detrimental effects of rubbing.
The use of a coating of abrasive material on the radially outer edges of a tip cap has been suggested as a partial solution to the above-mentioned problems. For example, such a tip cap is described in U.S. Pat. No. 4,169,020, assigned to the same assignee as the present invention. Although the abrasive material on such a tip cap cleans the inner surface of the shroud of deposits, thereby reducing rubbing and its adverse effects, when the abrasive coating is worn away, the tip cap is effectively transformed into a conventional, non-abrasive tip cap having the associated problems.
In view of the above problems, it is, therefore, a primary object of the present invention to provide a new and improved tip cap for a rotor blade which provides an effective close clearance seal between the tip of the rotor blade and the shroud.
Another object of the present invention is to provide a tip cap with a prolonged useful life for cleaning the inner surface of the shroud of deposits of material caused by rubbing.
Another object of the present invention is to provide a method for replacing a tip cap on a rotor blade.
Yet another object of the present invention is to provide a tip cap which reduces the thermal and shear stresses to the tip cap during rubbing.
Still another object of the present invention is to provide a tip cap having cooling arrangements which prolong useful tip cap life.